Stripline characteristic Impedance calculation using HFSS

Hello,

I am implementing Power divider using stripline. I have used lambda/4 = 75mm (1GHz) & air as a dielctric material. My question is that suppose i want to implement 57ohm line so to find out width ill first terminate it at 57ohm then see the reflected impedance if both are same then it is 57ohm characteristic line. Now if i terminate this line to 50ohm then it should reflect 64.98ohm but it is not reflecting. If i try to make 64.98ohm reflection by varing width then first case is not satisfy. Please if anybody worked on striplines reply try to solve my problem. I am doing simulation on HFSS11.

Thank You.